I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to novel and improved method and apparatus for detecting zero rate frames in a data transmission.
II. Description of the Related Art
Many modern day communications systems currently exist for transmitting data from a source device to a destination device. Among these systems, code division multiple access (CDMA) communications systems are efficient data transmission systems that employ spread spectrum techniques to utilize an entire available signal bandwidth. CDMA systems use other techniques to further enhance system capacity while providing the required level of performance. Such techniques include dynamic adjustment of the transmit power level and data transmission at a variable rate.
In CDMA systems, communication between users is conducted via one or more base stations. A first user on one mobile station communicates to a second user on a second mobile station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is then transmitted on the forward link of the same base station, or a second base station, to the second mobile station. The forward link refers to transmission from the base station to the mobile station, and the reverse link refers to transmission from the mobile station to the base station.
Data transmissions for CDMA systems occur in frames of data. To enhance system capacity, the rate of each frame can be selected from one of a number of possible rates (e.g., full, half, quarter, and eight rates), depending on the amount of data to be transmitted. For some CDMA systems, transmission occurs in specified (e.g., 20 msec) time intervals, with each interval comprising a single larger (20 msec) frame or a number of smaller (5 msec) frames. Each frame can include a data transmission or no data transmission. A frame with no transmission is commonly referred to as a zero rate (or empty) frame.
The variable and zero rate frames allow the CDMA system to increase capacity by decreasing the transmit power level, and thus reducing interference, when smaller amounts or no data is present for transmission. At the receiving device, a detection scheme is necessary to detect whether a frame was received correctly (i.e., a good frame) or received in error (i.e., an erased or bad frame), or whether no transmission occurred (i.e., a zero rate or empty frame). This information may be required, for example, to adjust the transmit power level at the transmitting source to maintain a specified level of performance.
As can be seen, techniques that can accurately identify zero rate frames are highly desirable.
The present invention provides novel and improved techniques for detecting zero rate frames in a received data transmission. Zero rate detection can be achieved using various methods. Typically, a quality metric is computed for a received frame that cannot be reliably decoded and compared against a threshold value. Based on the comparison result, the received frame is indicated as being either transmitted and received in error (i.e., erased or bad) or not transmitted at all (i.e., zero rate or empty). In accordance with different aspects of the invention, the threshold value can be: 1) selected based on the quality metrics computed for decoded frames, 2) selected based on the quality metrics computed for received frames identified as good, and 3) dynamically adjusted based on current information available at the receiver. These features increase accuracy in identifying zero rate frames by taking into account the operating conditions of the receiver.
An embodiment of the invention provides a method for identifying zero rate frames in a received data transmission. In accordance with the method, a modulated signal is received and demodulated in accordance with a particular demodulation format to generate demodulated symbols. The demodulated symbols are partitioned into a number of received frames. For each received frame, the symbols are decoded and certain decoding metrics (e.g., symbol error rate, CRC, and so on) are checked to determine the success of decoding. If decoding fails, or if a rate determination algorithm (RDA) needs to distinguish between zero rate and erased frames, a quality metric is computed and compared against a threshold value. The threshold value is selected based, in part, on the quality metrics of received frames. A particular received frame is identified as being a zero rate frame or not a zero rate frame based on the comparison. The method is advantageously used in a CDMA communications system.
The quality metric can relate to an energy of a received frame, a distance between a received frame and a codeword corresponding, to the received frame, or other metrics. The energy can be computed as a sum of square symbols for the received frame. The distance can be computed by decoding a received frame, re-encoding the decoded data (if a non-systematic code is used at the transmitting device), and performing a dot product of the received frame with the decoded or re-encoded frame. The threshold value can be selected based, in part, on the computed quality metrics of decoded frames identified as good frames, and can also be dynamically adjusted.
Another embodiment of the invention provides a receiver subsystem in a communications system. The receiver subsystem includes a demodulator coupled to a data processor. The demodulator receives and demodulates a modulated signal in accordance with a particular demodulation format to generate demodulated symbols. The data processor is configured to: 1) partition the demodulated symbols into a number of received frames, 2) compute a quality metric for each received frame, 3) compare the quality metric for a particular received frame against a threshold value, and 4) identify the particular received frame as being a zero rate frame or not a zero rate frame based on the comparison. The threshold value is selected based, in part, on the quality metrics of received frames.
The data processor can include: 1) a decoder that receives and decodes the received frames to generate decoded frames, 2) a CRC circuit that receives and checks the decoded frames to identify good frames among the decoded frames, and 3) an encoder that receives and re-encodes the decoded frames, or a combination thereof. The quality metric can relate to an energy, a distance, or other metrics of the received frame.
Yet another embodiment of the invention provides a receiver subsystem used in a CDMA communications system and operable to identify zero rate frames in a received data transmission. The receiver subsystem includes a demodulator, a decoder, a CRC circuit, and a metric calculation unit. The demodulator receives and demodulates a modulated signal in accordance with a particular demodulation format to generate demodulated symbols. The decoder receives the demodulated symbols as a plurality of received frames, and decodes the received frames into decoded frames. The CRC circuit receives and checks the decoded frames to identify good frames among the decoded frames. The metric calculation unit computes a quality metric for each of the plurality of received frames, compares the quality metric for a particular received frame against a threshold value, and identifies the particular received frame as being a zero rate frame or not a zero rate frame based on the comparison. The threshold value is selected based, in part, on the quality metrics of received frames.